In Pursuit of Pollution-Free Fuels

Carey P. Reid

This past December, Associate Professor of Chemistry Nelly Rodriguez learned in one very surprising instant that she and her fellow researchers were onto something important. She was scheduled to be the third speaker at a Materials Research Society conference held here in Boston. The title of her presentation was “Graphite Nanofibres: The Answer to Hydrogen Storage.” When the keynote address was delivered, there were twenty people in the room. The second speaker failed to show, so a coffee break was called, and Rodriguez was asked to set up her charts and slides. She did so and left the hall, a bit disappointed that her recent findings, which she felt were very exciting, would be shared with so few people. “But when I returned after the break,” she recalls with a modest smile, “there were three hundred people sitting in the room.”

The story actually begins nearly two decades ago, with the early research of Terry Baker, now professor of chemistry in the College of Arts and Sciences. Educated at Liverpool Polytechnic and the University of Wales, Dr. Baker is the recipient of scores of awards and the author of over two hundred publications. He is one of the most highly regarded experts in the field of catalysis. Years ago, he discovered graphite nanofibres while employed at the Atomic Energy Authority in Harwell, England. From the start, he knew that these fibres would be important someday, but the material was regarded by his supervisors as a nuisance material, a waste product of catalytic reactions whose production should be inhibited. Ironically, those microscopic fibres may soon become the key material for revolutionary new power sources that will very likely make petroleum obsolete as an automobile engine fuel.

Before moving to Exxon’s Corporate Research Laboratory in New Jersey, the English-born Baker worked at the Atomic Energy Authority in Harwell, England. His research there focused on controlled atmosphere transmission electron microscopy experiments to investigate various factors in the field of catalysis. Particular materials — catalysts — enable and stimulate predictable chemical reactions wherein the original catalyst remains unchanged but other materials undergo transformation. For example, the transformation of highly toxic carbon monoxide into carbon dioxide (and other gases) occurs regularly in the catalytic converters used in gasoline-powered vehicles.

While at Harwell, in addition to discover-ing the graphite nanofibres, Baker also helped pioneer developments in electron microscopy, a technical imaging process that allows the researcher to directly “observe” chemical reactions as they occur, and with breathtaking clarity. In time, he became increasingly intrigued with the altering “behavior” of particular catalysts when bonded to different support materials. To use the example of catalytic converters again, the catalyst is most often platinum particles attached to a support of aluminum oxide; those same platinum particles will change shape, however, when attached to different support materials, and the shape of the catalyst determines the character and type of chemical reactions it will promote.

Baker explains that his research came to focus on “how to manipulate the behavior of catalyzing metals to alter and control their chemical reaction with gases.” His expertise in catalysis and his mastery of the newer microscopy techniques made him a very attractive candidate for work in the energy industry. Exxon succeeded in enticing Baker to move across the Atlantic.

After eleven years with Exxon, Baker left in order to pursue a career in academia, first at Pennsylvania State University. “A researcher must be allowed to be a little nuts,” he explains. “There must be a visionary aspect to research, which only the university environment allows.” This past summer, Baker and Nelly Rodriguez, one of his top Penn State research colleagues and his wife of six years, were lured to Northeastern. “We were very impressed by Dean Lowndes, and in particular by what he was trying to achieve with research at the University,” Baker recounts. “We toured the new Egan Center as well, and we’ve since been allowed some say in the development of that wonderful new facility.”

Dr. Rodriguez, who studied at the Universidad Nacional de Colombia and the University of Newcastle, England, has “spun off” a particular area of research stimulated by Baker’s conviction that graphite nanofibres are more than a sooty by-product of catalytic reactions. Her work has led to an understanding of how these microscopic forms grow in highly graphitic forms. As the catalytic reaction proceeds, platelets of precipitated carbon are literally stacked below and above the metal particle; different metals produce different configurations of the platelets. In other words, each whiskerlike nanofibre is a highly configured arrangement of microscopic carbon wafers — some stacked like crackers, some slanted end to end like the herringbone pattern of a Harris tweed jacket, and some bent to form tubes. One of the consistent properties of these nanofibres is that the distance between the platelets is identical. More important, the microscopic widths of certain fibres are just large enough for hydrogen molecules to slip in but too narrow for larger molecules. Rodriguez wondered if these nanofibres could be used as “storage containers” for hydrogen molecules. Her discovery that they are very well suited for such an application has been the crux of all the recent excitement — particularly where hydrogen storage solves certain problems in another area of research and development, the fuel cell.

Fuel cell technology has been around for some time. Batteries, in all forms, are primitive fuel cells, and tiny graphite fuel cells keep our laptop computers’ clocks ticking and their bytes of basic-level memory intact. The direction of fuel cell technology, however, has been driven by the need to produce smaller, more powerful, and “cleaner” units (without, for example, the toxic mercury content of flashlight batteries). The Baker-Rodriguez research points the way to a formidable leap in fuel cell technology: if hydrogen, for example, can be stably stored in large quantities in a small unit, then applications only remotely envisioned just a few years ago are now wholly possible.

Baker, Rodriguez, and their doctoral researchers have mounted more than fifty experiments demonstrating that thirty liters of hydrogen gas can be stored in a single gram of carbon. Such vastly compressed storage makes it possible to use hydrogen as a fuel source in catalytic chemical reactions activated within a normal-size automobile. The catalysis would bond two atoms of the stored hydrogen to one atom of free-floating oxygen to produce a water molecule while releasing four electrons to power the vehicle’s electric engine. This is a fuel source that produces absolutely no heat and zero pollutants. The only by-product would be water.

The German auto manufacturer Daimler-Benz has produced a hydrogen-fueled car, the NECAR II, but the onboard fuel “cells” are simply old-style pressurized-gas cylinders. The heavy cylinders weigh 80 kilograms (kg) each, are pressurized to 245 atmospheres (atm), take up 280 liters of space, and will power a car for 156 miles, a negligible advance over the 70-mile range of current battery-powered electric cars. The NECAR II’s range will discourage all but the most environmentally conscious consumers; gasoline-powered cars can travel up to 450 miles between fill-ups — and there are very few places to “fill up” on hydrogen gas at the moment.

If nanofibres are used to hold the compressed gas, however, a remarkable new scenario unfolds. Rodriguez has determined that an approximately 85 kg fuel cell at half the atm and one-sixth the liter volume of the NECAR II’s fuel cylinders will contain so much pure hydrogen that the traveling range can be increased to an astounding 5,000 miles. This research, then, points to the very distinct possibility that within a few years we will be driving cars that produce no pollutants, whose energy by-product is simple water, and whose driving distance between “fill-ups” is from here to Los Angeles and back.

Other researchers have tried to use nanofibres for hydrogen storage, but none has reported the dramatic storage capacities — 30 liters of gas per gram of graphite — of Rodriguez’s team. Building on Baker’s extensive understanding of the materials’ properties, she has ascertained that charging the graphite with electrons allows the hydrogen molecules to squeeze between the platelets in layers. Furthermore, the squeezing reduces the vibrational activity of the hydrogen molecules, allowing more room for compression.

In a December 1996 article of New Scientist, Stephen Hill describes the new hydrogen storage process. The nanofibres are first “washed with acid to remove metal impurities from the catalyst particles, and then heated to over 900-degrees C and placed under a vacuum to remove any gases already clogging up the slits. Hydrogen is then pumped in at an initial pressure of around 120 atmospheres. The pressure must then be maintained at 40 atmospheres to keep the hydrogen in place, and the gas can be released by gradually reducing the pressure.” Because the nanofibre materials will cost less than one dollar per kilogram to produce and because each fibre can be reused several times, the potential storage process will be remarkably inexpensive. The results reported in the New Scientist article were also discussed by Baker and Rodriguez in a recent interview with Richard Black of the BBC’s World News Service, as part of the program New Ideas.

Rodriguez is involved in other projects as well, including research on the removal of radioactive metal contaminants from lakes and streams, the removal of contaminants from soil, and methods for collecting and storing methane, a relatively plentiful and clean-burning gas. It is her work on graphite nanofibres, however, described in ten publications to date, that has generated the most excitement. Now she receives calls from worried members of the petroleum industry and from elated officials of departments of defense on both sides of the Atlantic. In the spring of this year, Rodriguez will speak in Austria and then visit with representatives of Daimler-Benz and Bavarian Motor Works.

Terry Baker is a professor of chemistry, and Nelly Rodriguez is an associate professor of chemistry. Their research is funded by the Department of Energy and the National Science Foundation.